E-Ink is developing its own color technology, which uses side-by-side red, blue, and green filters. This means that, at any given time, each pixel might only be reflecting light from one third of its total area, which can compromise the brightness of the display.

HP hopes to deliver a brighter color reflective display technology by stacking red, green, and blue pixels in the same area. "If you want to show red, you can make the entire display red," says Taussig.

The challenge in stacking pixels is that light is lost as it travels into and out of the stack display. "If you get loss at each layer, you get a huge overall loss, so we're engineering the light path to prevent that," says Taussig. HP's best approach so far is to stack layers of red, green, and blue dye between electrically active mirrors that control whether or not light passes through each layer.

"The drawback is that it's complicated," says Taussig. With every layer that must be added during manufacturing, there's more potential for errors. So the company is also developing a single-layer multicolor reflective display that uses luminescent materials to harvest light and convert it into different colors, which are then re-emitted.

A potential advantage for HP is its association with Phicot, which already makes high-performance display backplanes by printing silicon on plastic using a roll-to-roll process. "We've got to get the glass out of there," Colaneri says.

Korean display company LG and Taiwanese company Prime View International are also printing silicon transistors on top of flexible materials, and both these companies have promised fully flexible displays in the coming year.

"Phicot's approach is completely different," says Colaneri. Instead of going through multiple rounds of etching to create a transistor array, which entails multiple opportunities for error, Phicot uses a single-step, three-dimensional lithography technique. Eliminating manufacturing steps is particularly important when working with plastic: if it bends or warps during the printing process, the layers won't line up with one another. If HP can clear these and other manufacturing hurdles, its reflective display technology could be appearing soon on a billboard near you.